Optics Topics

Abaxial: Rays of light which do not coincide with the optical axis of a lens system.
Abbe Number: The ratio of refractivity to dispersion in an optical medium.
Abbe Refractometer: An instrument for measuring directly the refractive index of liquids, minerals and gemstones.
Aberration: A defect in a mirror or lens causing light rays from a single point to fail to focus at a single point in space.
Absorb: To transform radiant energy into a different form, with a resultant rise in temperature.
Absorption: Transformation of radiant energy to a different form of energy by the interaction of matter, depending on temperature and wavelength.
Absorption Coefficient: The fraction of sound energy that is absorbed at any surface.
Accessible Emission Level: Magnitude of accessible laser radiation of a specific wavelength or emission duration at a particular point.
Achromatic Lens: Lens for which all light colours have the same focal length.
Actinic: Pertaining to electromagnetic radiation capable of initiating photochemical reactions.
Actinic Ray: A ray of light of short wavelengths, as ultraviolet or violet, that produces photochemical effects.
Adaptive Optics: A technique that uses deformable mirrors on a telescope to correct the blurring caused by turbulence in the atmosphere.
Additive Primaries: Red, green, and blue are the primary colours of light from which all other colours can be made.
After Image: An image seen after the eye′s retina has been exposed for a time to an intense or stationary light source.
Airy Disc: A pattern of illumination caused by diffraction at the edge of a circular aperture.
Alidade: That part of an optical measuring instrument comprising the optical system, indicator, vernier, etc.
Anamorphic: Distorted, as in an optical system with different magnification levels or with focal lengths perpendicular to the optical axis.
Angle of Incidence: Angle between direction of motion of waves and a line perpendicular to the surface the waves are striking.
Angle of Reflection: Angle between direction of motion of waves and a line perpendicular to surface the waves are reflected from.
Angle of Refraction: The angle at which a refracted ray of energy leaves the interface at which the refraction occurred.
Angstrom: It is an old unit used for measuring the wavelength of electromagnetic radiation including visible light and X-rays.
Anti Reflection Coating: A thin layer of material that, when applied to a lens, increases its transmittance and reduces its reflectance.
Aperture: An opening through which radiation can pass.
Apostilb: A unit of surface luminance used when defining diffusing surfaces equal to 1 lumenm^-2
Arago Spot: A bright spot that appears in the shadow of a uniform disc being backlit by monochromatic light emanating from a point source.
Aspherical: Not spherical. To reduce spherical aberration, a lens may be altered slightly so that one or more surfaces are aspherical.
Astigmatism: An aberration, or defect, in a mirror or lens that causes the image of a point to spread out into a line.
Atmospheric Windows: The spectral bands in which the atmosphere least affects the transmission of radiant energy.
Autocollimator: A single instrument combining the functions of a telescope and a collimator to detect small angular displacements of a mirror by means of its own collimated light.
Autokinetic Illusion: The illusion of a fixed object or light moving when gazed at steadily.
Axial Flow Laser: An axial flow of gas is maintained through the tube to replace those gas molecules depleted by the electrical discharge used to excite the gas molecules to the lasing.
Axicon Lens: A conical lens which, when followed by a conventional lens, can focus laser light to a ring shape.
Back Focal Length: The distance between the last surface of a lens to its back focal plane.
Bandpass Filter: An electronic network which transfers through it a specific band of signal frequencies.
Bandwidth: The bandwidth of a filter is the separation between the lower and upper frequencies at which the amplitude of a sinusoidal signal is attenuated by a factor of 2.
Beam Splitter: An optical device using controlled reflection to produce two beams from a single incident beam.
Bending: The ratio of the curvatures of a lens′s two refracting surfaces.
Bi-Concave Lens: Having two outer surfaces that curve inward.
Bi-Convex Lens: Having two outer surfaces that curve outward.
Birefringence: The separation of a beam of light into two beams as it passes through a doubly refracting material or object.
Blur Circle: The image of a point-source object formed by an optical system on its focal surface.
Brightness: The visual sensation of the luminous intensity of a light source.
Broadband Coating: A multi-layer antireflection coating with reduced reflection over a broad spectral band.
Camera: An instrument for taking photographs consisting of a lightproof box, shutter, adjustable aperture and a lens through which an image is focussed on a light sensitive film or sensor.
Camera Lens: An arrangement of one or more pieces of optical glass designed to collect and focus rays of light to form a sharp image on the film.
Camera Obscura: A room with a small hole in one wall used by artists to produce images.
Candela: The SI unit of luminous intensity.
Carbon Dioxide Laser: A widely used laser in which the primary lasing medium is carbon dioxide gas.
Cassegrain Telescope: Two-mirror reflecting telescope.
Catadioptric: An optical system containing both reflective and refractive elements.
CCD: Abbreviation of Charge Coupled Device.
CCTV Lens: Abbreviation of Closed Circuit Television Lens.
cd: An abbreviation of candela, the SI unit of luminous intensity.
Centre Wavelength: For filters and coatings, the average of the wavelength values at the half-power points of the transmission band.
Charge Coupled Device: A self-scanning semiconductor imaging device which uses metal-oxide-semiconductor, surface storage and information transfer technologies.
Chromatic Aberration: A defect in lenses that causes different colors (wavelengths) of light to have different focal lengths.
Closed Circuit Television Lens: A family of compound lenses which offer exceptionally high resolution, short focal length and color imaging at fast lens apertures, such as required in the television industry.
C-Mount: CCD camera lens connector thread 1"-32UN2A with a standard focal distance of 17.526mm.
CO₂ Laser: Abbreviation of Carbon Dioxide Laser.
Coherent: A property of two or more sources of waves that have the same wavelength and maintain constant phase differences.
Coherent Anti-Stokes Raman Scattering: A phenomenon observed in the scattering of light as it passes through a transparent medium. The light undergoes a change in frequency and a random alteration in phase due to a change in rotational or vibrational energy of the scattering molecules.
Collimated Light: Light rays that are parallel.
Collimation: The alignment of the direction of the photons, so the beam of radiation can be directed at a well-defined part of a target material.
Collimator: A mechanical device installed along the trajectory of a beam to reduce the size of the beam.
Colour: A quality of light, depending on its wavelength. Spectral colour of an emission of light is its place in the rainbow spectrum.
Complementary Colours: For lights, two colours that combine to form white.
Concave Lens: Lens thinner in centre than edges; a diverging lens.
Converging Lens: Lens that causes light rays to converge; usually a convex lens.
Convex Lens: Lens that is thicker in center than at edges.
Convolution: Convolution meaning ..intertwined, coiled�
Critical Angle: The minimum angle of incidence for which total internal reflection occurs.
CS-Mount: CCD camera lens connector thread 1"-32UN2A with a standard focal distance of 12.5mm.
Curvature: Departure from flatness of a surface. Defined as the reciprocal of the radius of curvature.
Depth of Field: The distance along the optical axis through which an object can be located and clearly defined when the lens is in focus.
Depth of Focus: The distance along the optical axis through which an image can be clearly focused.
Dextrorotatory: A chiral molecule which rotates plane-polarized light to the right.
Diastereomers: Optical isomers that are not mirror images of each other.
Diffraction: The spreading of waves passing through an opening or around a barrier.
Diffraction Grating: Grating device that splits light into a spectrum of the component wavelengths.
Diffraction Limited: Describes an optical system in which the quality of the image is determined only by the effects of diffraction and not by lens aberrations.
Diffuse Reflection: The reflection of rays from a rough surface. The reflected rays do not leave at fixed angles.
Dispersion: The spreading of light into a spectrum of colour. The variation in the speed of a periodic wave due to its wavelength or frequency.
Dots Per Inch: A unit used to measure the resolution, or sharpness of an image.
dpi: Abbreviation of Dots Per Inch.
Electromagnetically Induced Transparency: A technique that makes normally opaque substances transparent to light by applying another source of light, such as a laser beam, that interferes with the opaque substance′s absorption of light.
Emission Spectrum: The collection of discrete wavelengths emitted by atoms that have been excited by heating or by electric currents.
Emissivity: The ratio of the radiant energy emitted by a any source to that emitted by a blackbody at the same temperature.
EUV: Extreme Ultraviolet, a portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.
Extended Source: An extended source of radiation can be resolved into a geometrical image in contrast with a point source of radiation, which cannot be resolved into a geometrical image.
Extreme Ultraviolet: A portion of the electromagnetic spectrum from approximately 100 to 1000 angstroms.
Farsightedness: Hyperopia. Images of distant objects are formed beyond the retina.
Fata Morgana: A complex mirage display that involves multiple images.
Fermat′s Principle: The path taken by a ray of light between any two points in a system is always the path that takes the least time.
Fibre Coating: Material immediately covering the optical fibre to preserve the integrity of the fibre.
Fibre Optics: The transmission of radiant energy through transparent fibres of glass, plastic or fused silica.
Field of View: The maximum visible space seen through an optical instrument or lens.
Filter: Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiation of certain frequencies while passing others.
First Order Optics: Optical characteristics limited to infinitesimally small pencils of light.
Flicker: Impression of fluctuating luminance or colour.
Fluid Opacity: Related to the ability of a fluid to pass light.
Focal Length: The distance from a mirror or the centre of a lens to its focal point.
Focal Plane: A surface upon which the image of all points in the field of view of an optical instrument is created.
Focal Point: The location at which a mirror or a lens focuses rays parallel to the optic axis or from which such rays appear to diverge.
Focus: Point at which converging rays meet and at which a clearly defined image can be obtained.
Foot Candle: A measure of light intensity on a surface being illuminated.
Fovea: The central portion of the retina where visual acuity, or the ability to distinguish small objects and details, is greatest. Only about half a millimetre in diameter, the fovea is the retina′s "rod-free zone" and is densely packed with cones.
Fraunhofer Diffraction: Fraunhofer diffraction describes the diffraction pattern of electro-magnetic radiation observed in the far field from a slit or series of slits (i.e. grating).
Fresnel Reflection: Reflection occurring at the planar junction of two materials having different indices of refraction.
f-stop: The ratio of the focal length of the lens to diameter of the aperture.
Fused Quartz: The glassy isotropic form of quartz.
Fused Silica: SiO₂
The glassy isotropic form of quartz.
Gas Laser: A type of laser in which the laser action takes place in a gas medium.
Gaussian Optics: Optical characteristics limited to infinitesimally small pencils of light.
Geometric Optics: That branch of optics dealing with the tracing of ray paths through optical systems.
Graded Index Fibre: An optical fibre whose core refractive index decreases from its centre out to its edge.
Group Index: The ratio of the vacuum velocity of light to the group velocity in the medium.
Helium-Neon Laser: A laser in which the active medium is a mixture of helium and neon.
HeNe Laser: Abbreviation of Helium-Neon Laser.
Hologram: A three-dimensional record of visual information.
Holography: The photographic process for producing three-dimensional images.
Hyperopia: Farsightedness. Images of distant objects are formed beyond the retina.
Illuminance: This is defined as the luminous flux arriving at a surface that is perpendicular to it per unit area.
Image Circle: The circular image field over which image quality is acceptable, can be defined in terms of its angular subtense.
Image Inversion: Change in the orientation of an image in one meridian.
Image Plane: The plane perpendicular to the optical axis at the image point.
Incident Wave: The wave that strikes the surface of a medium.
Index Matching Gel: Material whose index of refraction is almost equal to that of the fibre core and is used to minimize Fresnel reflections.
Index of Refraction: This is the ratio of the velocity of light in free space to that in the material.
Inferior Mirage: Mirage in which the inverted image is below the normal one.
Interference Filter: A filter which controls the spectral composition of transmitted energy by interference.
Inversion: Referring to something being upside down.
Ion Laser: A type of laser employing a very high discharge current, passing down a small bore to ionize a noble gas such as argon or krypton.
Iris: The pigmented fibrovascular tissue known as a stroma. It is the most forward portion of the eye and the only one seen on superficial inspection.
KDP: K.D.P. (Potassium D Phosphate) is a crystal often used in non-linear optics within Pockles Cells, Kerr Cells to vary the polarisation and phase of light. Also used as a frequency doubling crystal in lasers.
Kerr Cell: A Kerr Cell is most commonly used as a Q-Switch device in pulsed lasers
Kerr Effect: The ability of certain substances to differently refract light waves whose vibrations are in different directions when the substance is placed in an electric field.
Lamberts Laws: The illuminance on a surface illuminated by light falling on it perpendicularly from a point source is proportional to the inverse square of the distance between the surface and the source.
Laser: An acronym for light amplification by stimulated emission of radiation.
Laser Diode: A laser which uses a forward biased semiconductor junction as the active medium.
Lens: A lens is an optical component whos surface curvature and refractive index are designed such that the direction of light incident upon it is controlled.
Lens Mount: A mechanical and often also electrical interface between a photographic camera body and a lens.
Lens Thread M39: Also known as the Leica L-Mount. This is 39mm x 26tpi.
Lens Thread M42: This is an metric 42x1.0mm thread used for single reflex cameras before the bayonet couplings became commonplace.
Lens Thread T2: 42mm x 0.75mm lens adaptor thread used by Tamron.
Levorotatory: A chiral molecule which rotates plane-polarized light to the left.
Light: Electromagnetic radiation that is visible to the unaided human eye.
Light Beam: A collection of rays that may be parallel, convergent, or divergent.
Light Ray: A line that represents the path of light in a given direction.
Light Scattering: When light passes through a medium that is not perfectly homogeneous, the irregularities in the medium scatter some of the light in all directions.
Limit of Resolution: The limit to the performance of a lens imposed by the diffraction pattern resulting from the finite aperture of the optical system.
Linke Turbidity Factor: A measure of atmospheric turbidity, equal to the ratio of total optical depth to the Rayleigh optical depth.
lm: An abbreviation of lumen, the derived SI unit of luminous flux.
Lossy Medium: A medium which absorbs or scatters radiation passing through it.
Lumen: The derived SI unit of luminous flux.
lux: An abbreviation of lumen m^-2, the metric unit of illuminance.
lx: An abbreviation of lux that itself is an abbreviation of lumen m^-2, the metric unit of illuminance.
Macrobending: Curvature of a fibre which causes loss of light.
Magnification: The enlargement of an object by an optical instrument.
Malus′ Law: Malus′ law defines the transmitted intensity through 2 polarisers for a given angle between the transmission directions of said polarisers.
Microbending: Microscopic bends and bumps in the fibre which cause loss of light by scattering.
Mirage: An optical effect that produces an image that looks like it has been reflected from the surface of a body of water.
Mode Coupling: Energy exchange between modes of light propagating in an optical fibre.
Moire Fringes: A pattern formed by the interference between two regular sets of divisions.
Monochromatic: Radiation that has a single wavelength.
Multi Mode Fibre: Optical fibre supporting more than one propagating mode.
Myopia: Nearsightedness. Images of distant objects are formed in front of the retina.
Nd YAG Laser: Abbreviation of Neodymium Yttrium Aluminum Garnet Laser.
Nearsightedness: Myopia. Images of distant objects are formed in front of the retina.
Neodymium Yttrium Aluminum Garnet Laser: A synthetic crystal used as a laser medium to produce 1064 nm light.
Neutral Density: An optical coating that appears grey to the eye and has a flat absorption curve throughout the visible spectrum.
Nonluminous Bodies: Objects that either reflect or diffuse light that falls upon them.
Objective: The optical element which receives light from the object and forms the first or primary image in telescopes, microscopes and other optical systems.
Ocular: Pertaining to or in relation with the eye.
Oculogyric: Referring to movements of the eyes.
Opacity: The condition of being non-transparent.
Opaque: Substances that do not transmit (pass) any light rays.
Optic Axis: A line passing through the center of a curved mirror and the center of the sphere from which the mirror is made.
Optical Activity: A substance that is capable of rotating plane-polarized light.
Optical Axis: A line passing through the centres of curvature of a lens or series of lenses in an optical system.
Optical Cavity: Space between the laser mirrors where lasing action occurs.
Optical Coupler: A coupler composed of an LED and a photodiode and contained in a lightconducting medium.
Optical Filter: A device used to reject certain frequencies of light.
Optical Flat: A piece of glass with one or both surfaces polished flat.
Optical Illusion: A drawing or object that appears to have an effect that it does not really have, such as when a flat painting seems to have three-dimensional depth.
Optical Interference: The interaction of two light waves on the total intensity of light.
Optical Pumping: The excitation of the lasing medium by the application of light rather than electrical discharge.
Optical Pyrometer: An instrument for measuring the temperature of heated material.
Optical Transmission: To admit the passage of light through a medium; light not reflected back to its source is transmitted through the medium.
Parabolic Reflector: An antenna reflector in the shape of a parabola.
Parallax: Change in the apparent position of objects when viewed from two widely separated positions.
Paraxial Optics: Optical characteristics limited to infinitesimally small pencils of light.
penumbra: The transition region between the darkest shadow and full brightness.
Perceived Colour: Also known as visual colour is the quality of light emission as conveyed by the human eye, combining the impressions of 3 types of light-sensitive cells which the eye contains.
Phase: Phase identifies the position at any instant which a periodic wave occupies in its cycle. A portion of a material system whose properties and composition are homogeneous and which is physically distinct from other parts of the system.
Photoluminescence: Luminescence caused by absorption of optical radiation.
Photometer: An instrument which measures luminous intensity.
Photons: Massless packet of energy, which behaves like both a wave and a particle.
Picture: Visual representation of something.
Pinhole: A small, sharp-edge hole without a lens which can function as an aperture or eye lens.
Pixel: Single picture element of a detection device.
Plane Wave: Wave in which the wavefronts are everywhere parallel planes normal to the direction of propagation.
Pockels Cell: A Pockels Cell is a device which contains a photo refractive crystal. When a voltage is applied to this crystal it can change the polarisation or phase of the light beam.
Point Source: Ideally, a source with infinitesimal dimensions.
Poisson Spot: Poisson originally predicted the existence of the arago spot, and used the prediction to demonstrate how the wave theory of light must be in error to produce such a counterintuitive result.
Polarized: A property of a transverse wave when its vibrations are all in a single plane.
Power of a Lens: The power of a lens is defined as the reciprocal of its focal length, in metres.
Prism: A transparent optical element with at least two polished planes inclined toward each other, from which light reflects or through which it is refracted.
Propagation: Waves traveling through a medium.
Q-Switch: Used to build up a high population inversion within pulsed lasers.
Q-Switched Laser: A laser which stores energy in the laser media to produce extremely short, extremely high intensity bursts of energy.
Radiant Exposure: The total energy per unit area incident upon a given surface.
Radiosity Equation: Used in the construction of realistically lit scenes in computer graphics.
Ray Tracing: The calculation of the propagation, reflection and refraction of rays from source to receiver. This technique is used extensively within the optics and acoustics industry.
Rayleigh Criterion: A criteria that gives an approximation for how finely a set of optics may be able to distinguish.
Real Image: An image formed by the convergence of light.
Reflecting Telescope: A type of telescope using a mirror as the objective.
Refracting Telescope: A type of telescope using a lens as the objective.
RGB: Abbreviation of Red, Green, Blue.
Schlieren: An optical technique that detects density gradients occurring in a fluid flow.
Shadow: Darkness in a region, caused by an obstruction between the source of light and the region.
Shadowgraph: A picture or image in which steep density gradients in the flow about a body are made visible.
Shutter: A device for controlling the amount of time a light-sensitive medium is exposed to light.
Single Mode Fibre: Optical fibre supporting only one mode of light propagation.
Snells Law: Snells law defines the angle of refraction of a light beam when it passes from one medium to another.
Solar Air Mass: The optical air mass penetrated by light from the sun for any given position of the sun in the sky.
Spectrometry: Breaking up white light into its constituent wavelengths and measuring them on a calibrated scale.
Stroboscope: An instrument that allows viewing of rotating or reciprocating objects by producing the optical effect of a slowing down or stopping motion.
Telecentric Lens: A multi-component system whose aperture stop is located at the front focus so that the chief rays are parallel to the optical axis in image space.
Telescope: Instrument used to focus electromagnetic radiation into an image.
Thermal Imaging Camera: A camera that detects infrared radiation and renders it as visible light thus allowing you to see the temperature of objects in the environment.
Thermography: The detection and display of infrared radiation.
Translucent: Those substances, such as frosted glass, through which some light rays can pass but through which objects cannot be seen clearly.
Transparent: Those substances, such as glass, that pass almost all of the light waves falling upon them.
Transverse Electromagnetic Mode: The radial distribution of intensity across a beam as it exits the optical cavity.
Transverse Ray Error: Errors, or departures from ideal, measured in a direction perpendicular to the optical axis.
Tripod Mounts: The thread on tripod mounts is 1/4"-20 UNC, which is the smaller more common size. For larger cameras the 3/8"-16 UNC thread is used.
Truncation Ratio: The dimensionless ratio of the Gaussian beam diameter at the 1/e² intensity point to the limiting aperture of the lens.
Tunable Dye Laser: A laser whose active medium is a liquid dye, pumped by another laser or flashlamps, to produce various colours of light.
Tunable Laser: A laser system that can be tuned to emit laser light over a continuous range of wavelengths or frequencies.
UV: Abbreviation of ultraviolet.
Vignetting: The gradual reduction of image illuminance with an increasing off-axis angle, resulting from limitations of the clear apertures of elements within an optical system.
Vitreous Silica: The glassy isotropic form of quartz.
Wavefront Deformation: Departure of a wavefront from ideal (usually spherical or planar) caused by surface errors or design limitations.
Wavelength Dispersion: Dispersion cause by the dependence of the phase and group velocities on wavelength due to geometric properties of the waveguide.
Window: A piece of glass with plane parallel sides which admits light into or through an optical system and excludes dirt and moisture.
YAG: A widely used solid-state crystal which is composed of yttrium and aluminium oxides which is doped with a small amount of the rare-earth neodymium.
Young′s Double-Slit Experiment: A famous experiment which shows the wave nature of light.
Zenith Telescope: Instrument for observing stars near the zenith.

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Optics Topics